Introduction to GB/T 193-2003
GB/T 193-2003 specifies the diameters and pitches for general purpose metric screw threads, also known as ordinary threads. This standard is essential for ensuring compatibility and interchangeability in mechanical engineering applications, such as fasteners, machinery components, and assembly systems. It defines three series of diameters and distinguishes between coarse and fine pitches, with priorities to guide selection for optimal performance and manufacturability.
The standard aligns with international practices, drawing from ISO 261, and is widely used in industries including automotive, aerospace, and manufacturing. Proper adherence minimizes failures due to mismatched threads, enhances load distribution, and supports efficient production processes.
Diameter Series
Diameters are categorized into three series: 1st series (preferred), 2nd series (secondary), and 3rd series (least preferred). Prioritize the 1st series for standard applications to promote uniformity. The series range from M1 to M300, covering small precision threads to large structural ones.
- 1st Series: Primary choice for most designs, e.g., M1, M1.2, M1.6, M2, M2.5, M3, M4, M5, M6, M8, M10, M12, M16, M20, M24, M30, M36, M42, M48, M56, M64, M72, M80, M90, M100.
- 2nd Series: Used when 1st series is unsuitable, e.g., M1.1, M1.4, M1.8, M2.2, M3.5, M4.5, M7, M9, M11, M14, M18, M22, M27, M33, M39, M45, M52, M60, M68, M76, M85, M95, M105.
- 3rd Series: Avoid unless necessary, e.g., M5.5, M15, M17, M25, M26, M28, M32, M35, M38, M40, M50, M55, M58, M62, M65, M70, M75, M78, M82.
Selection guidance: Start with the 1st series to reduce inventory and tooling costs. For specialized needs, such as high-strength or space-constrained applications, consult the 2nd or 3rd series.
Pitch Series: Coarse and Fine Threads
Pitches are divided into coarse (primary for general use) and fine (for applications requiring higher tensile strength or vibration resistance). Coarse pitches offer easier assembly and are less prone to cross-threading, while fine pitches provide better adjustment and load-bearing in compact designs.
Key pitches include:
- Coarse pitches: Standard for each diameter, e.g., 0.25 mm for M1, increasing to 6 mm for larger diameters like M64.
- Fine pitches: Prioritized from 8 mm down to 0.2 mm, with availability varying by diameter.
Avoid pitches in parentheses where possible, as they may complicate manufacturing. For example, fine pitches like 1.25 mm are limited to specific diameters.
Priority Levels for Pitches
Pitches have priority degrees for fine threads, guiding selection:
- 8 mm: Highest priority for large diameters (e.g., M70 and above).
- 6 mm: Common for mid-to-large sizes (e.g., M56 to M300).
- 4 mm: For diameters from M42 upwards.
- 3 mm: Widely used from M36 to M300.
- 2 mm: For M18 to M52.
- 1.5 mm: From M10 to M60.
- 1.25 mm: Limited, e.g., M8, M10, M12.
- 1 mm: From M6 to M33.
- 0.75 mm: From M5.5 to M10.
- 0.5 mm: From M3.5 to M5.5.
- 0.35 mm: For small diameters like M2.5 to M3.5.
- 0.25 mm: For M2 to M2.2.
- 0.2 mm: For M1 to M1.8.
Higher priorities ensure better standardization and availability. For coarse threads, the single specified pitch is always preferred.
Detailed Diameter and Pitch Table
The following table summarizes diameters, series, coarse pitches, and available fine pitches per priority. Use it for precise selection in designs.
| Diameter | Series | Coarse Pitch | Fine Pitch (8 mm) | Fine Pitch (6 mm) | Fine Pitch (4 mm) | Fine Pitch (3 mm) | Fine Pitch (2 mm) | Fine Pitch (1.5 mm) | Fine Pitch (1.25 mm) | Fine Pitch (1 mm) | Fine Pitch (0.75 mm) | Fine Pitch (0.5 mm) | Fine Pitch (0.35 mm) | Fine Pitch (0.25 mm) | Fine Pitch (0.2 mm) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| M1 | 1st | 0.25 | – | – | – | – | – | – | – | – | – | – | – | – | 0.2 |
| M1.1 | 2nd | 0.25 | – | – | – | – | – | – | – | – | – | – | – | – | 0.2 |
| M300 | 2nd | – | 8 | 6 | 4 | 3 | – | – | – | – | – | – | – | – | – |
Notes: ① For spark plugs in engines only. ② For bearing lock nuts only. Prioritize 1st series diameters, then 2nd, then 3rd. Avoid pitches in parentheses if possible.
Applications and Best Practices
In practice, select threads based on load requirements, material strength, and environmental factors. For high-vibration settings, fine pitches are recommended. Always verify tolerances per GB/T 197 for mating parts. This standard supports sustainable design by standardizing components, reducing waste in production.
- Automotive: Use M10x1.25 for spark plugs (note ①).
- Machinery: M35 for bearing lock nuts (note ②).
- General: Coarse pitches for quick assembly.
Veelgestelde vragen (FAQ)
- What is the difference between coarse and fine pitches in GB/T 193-2003?
- Coarse pitches are standard for general use, providing easier installation and resistance to stripping. Fine pitches offer finer adjustment, higher strength per unit length, and better vibration resistance, ideal for precision or high-load applications.
- How do I choose the appropriate diameter series?
- Always start with the 1st series for standardization. Use 2nd series if the 1st does not meet dimensional needs, and 3rd only as a last resort to avoid non-standard tooling and supply issues.
- Are there restrictions on certain pitches or diameters?
- Yes, for example, M12x1.25 is noted for specific uses, and pitches in parentheses should be avoided to simplify manufacturing. M35 is limited to bearing lock nuts.
- How does this standard relate to international equivalents?
- GB/T 193-2003 is equivalent to ISO 261, ensuring global compatibility. Differences may exist in tolerance classes, so cross-reference with ISO for international projects.
- What materials are suitable for these threads?
- Common materials include carbon steel, stainless steel, and alloys. Select based on corrosion resistance and strength; for example, use AISI 304 for corrosive environments with fine pitches to maintain integrity.
- Can I use fine pitches for all applications?
- No, fine pitches require precise alignment and are more susceptible to damage during assembly. Reserve them for applications needing high tensile loads or where space is limited.
